Structural Characteristics of Natural-Gas-Vehicle-Aged Oxidation Catalyst

Deactivation of the natural-gas-vehicle-aged Pt/Pd oxidation catalyst supported on gamma-alumina-based washcoat was studied by electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, and catalytic activity measurements. Significant structural changes were detected in the used catalyst compared to the fresh one. Grain size of the washcoat had grown but its structure had remained the same, gamma-alumina, as in the fresh catalyst. Sintering of the noble metal particles had occurred, particle sizes varied from similar to 5 up to similar to 100 nm. Decomposition of palladium oxide and platinum oxide to metallic Pd and Pt occurred followed by the formation of the bimetallic Pt/Pd crystals. Also reformation of palladium oxide was detected. In addition, the inlet part of the used catalyst was totally covered by a poisoning layer. Due to these structural changes and poisoning, the activity of the vehicle-aged catalyst had decreased significantly compared to the fresh one. All the changes were stronger in the inlet than in the outlet part of the used catalyst indicating higher operating temperature and more extensive thermal deactivation and poisoning in the inlet than in the outlet part of the converter.